Solar cells using a chalcopyrite compound semiconductor such as CIS or CIGS have the highest conversion efficiency in thin-film solar cells, and are extensively studied. It is known that in a chalcopyrite compound semiconductor solar cell, an alkali metal is added to a compound semiconductor layer to improve conversion efficiency (so called an “alkali effect”).
For example, when a compound semiconductor solar cell including a metal layer and a compound semiconductor layer on a glass substrate is prepared, use of a soda lime glass as a substrate can enhance efficiency because sodium atoms are diffused into the compound semiconductor layer through the metal layer from the soda lime glass.
On the other hand, there is growing interest in compound semiconductor solar cells using a film substrate in terms of weight reduction and flexibility. Since CIS and CIGS are formed at a high temperature, a film substrate is required to have high heat resistance. Accordingly, a polyimide film is preferably used as a flexible substrate for a compound semiconductor solar cell.
For example, Patent Document 1 discloses a method in which a flexible compound semiconductor solar cell is prepared by forming a metal layer and a compound semiconductor layer on a polyimide film having high heat resistance and high dimensional stability. Specifically, a method for preparing a polyimide film as a substrate is disclosed in which a polyimide precursor (polyamide acid) solution is applied onto a support base, and heated at 200° C. or lower to form a self-supporting gel film, and the gel film is separated from the support base, and then subjected to a heating treatment at about 500° C. under no stress.
Patent Document 1 discloses that generation of cracks in a metal layer serving as an electrode and a semiconductor layer, and peeling of these layers from the substrate can be suppressed, when the metal layer is formed on a polyimide film B-surface that is in contact with a support base in coating of the polyimide film, and a chalcopyrite compound semiconductor layer is formed on the metal layer. According to Patent Document 1, the reason for this is that B-surface that is in contact with a support base during coating of the film has a strong surface structure, since volatilization of a solvent hardly occurs on the B-surface as compared to an air-surface (A-surface) so that imidization of a polyamide acid easily proceeds on B-surface.
An alkali metal diffuses from a substrate into a compound semiconductor layer when a soda lime glass or the like is used as the substrate as described above, whereas it is necessary to add an alkali metal in a compound semiconductor layer when a polyimide substrate is used. Patent Document 1 (see paragraph [0119]) proposes a method in which an alkali metal-containing layer is formed on a metal electrode layer, and a compound semiconductor layer is formed thereon.
Patent Document 2 discloses insertion of a silicate layer of soda lime (Na2.CaO.5SiO2) or the like between a polyimide substrate and a metal layer. In this method, heating during formation of a compound semiconductor layer causes an alkali metal in the silicate layer to pass/diffuse through a metal electrode layer, and is added to the compound semiconductor layer.